EPID (Electronic Portal Imaging Device) is ubiquitous in the radiation therapy clinic and provides a powerful and flexible tool to collect and process data in a quantitative manner to improve treatment accuracy for virtually any treatment site. This study provides an overview of the clinical implementation process for effective use of EPID. A proper control of the geometrical accuracy of treated patients during radiotherapy results in higher quality of treatment and may lead to the increase in therapeutic gain. In this study an evaluation of an electronic portal imaging was made in the following aspects: the quality of images, the estimation of prolongation of the treatment, corrections introduced after corrections.